ABSTRACT Unlike most carcinogens, which have been reduced or removed once recognized, ionizing radiation exposure has increased in the USA over the last 3 decades. The typical American's annual radiation dose has doubled over that period primarily due to increasing exposure to radiation from diagnostic medical procedures, such as CT exams. The doses from such procedures are typically quite low, but may be repeated over time and increase over adulthood. The aim of this project is to strengthen understanding of cancer risk associated with repeated, low dose exposure to ionizing radiation in adulthood. Among the most promising epidemiological studies of cancer risk following low dose ionizing radiation exposures are cohort studies of people who have been individually monitored for radiation exposure through personal radiation dosimeters. We have assembled an international cohort of 308,000 radiation dosimeter-monitored workers from some of the world's most informative cohorts in the United Kingdom, France, and USA. Here we propose major updates of each these cohorts and an innovative set of analyses to directly address questions relevant to radiation protection and decision making regarding low dose rate external ionizing radiation exposures. Specifically, we propose to assess: 1) cancer site-specific ionizing radiation risks; 2) age-related changes in susceptibility to low dose ionizing radiation; 3) persistence with time since exposure in excess relative rate of cancer among older adults, accounting for competing risks; and, 4) population-level estimates of the impact of low dose ionizing radiation exposures on cancer risk. The proposed study is likely to exert a sustained influence on the field and make a major contribution to national and international radiation protection recommendations. The evidence from the proposed study will help to inform cost-benefit assessments, and understanding of the role of increasing exposures to ionizing radiation on cancer rates.